A typical example of a technology for effecting optical display in a high temperature environment using a liquid crystal cell is a liquid crystal projector which illuminates the back of a liquid crystal panel comprising, for example, a liquid crystal cell and a polarizer plate to form a projected image of the liquid crystal panel, and projects a magnified image of the liquid crystal panel by an optical system as a projection means.
Hereinafter, the structure and problems with the liquid crystal projector ordinarily known in the art will be explained. FIG. 1 is a schematic view of the known liquid crystal projector described in Japanese Unexamined Patent Publication (Kokai) No. 4-27929, and an air inlet is shown disposed at a part of a housing and a dust-proofing filter 401 is added to this air inlet.
In the liquid crystal projector shown in FIG. 1, since transmissivity of a polarizer plate 103 is low, about 60% of the light leaving illumination means 203, comprising light source means 201 and a condenser lens 202, is converted to heat at the polarizer plate 103.
Particularly in the liquid crystal projector, exothermy in the polarizer plate 103 becomes extremely high because luminance of a lamp as light source means 201 of the illumination means is high. Accordingly, a change in temperature characteristics and deterioration of the liquid crystal and of the polarizer plate occur due to this high temperature, and a drop in image quality of the projected image and a reduction in the service life of the liquid crystal cell 108 occur due to the high temperature.
Various technologies have been developed in the past to solve the temperature problems described above.
For example, Japanese Unexamined Utility Model Publication (Kokai) No. 2-69316 and Japanese Unexamined Patent Publication (Kokai) No. 4-27929 described above propose a solution. However, the liquid crystal projector shown in FIG. 1 employs a construction wherein illumination means 203 comprising mainly a light source lamp 201, a liquid crystal cell 108, and an optical system 300 comprising mainly a projection lens are accommodated in a casing 500 having a sealed structure and equipped with a cooling fan 400, a polarizer plate 103 and a liquid crystal cell 108 are spaced apart from each other, and the cooling fan 400 cools the inside of the liquid crystal projector so as to cool the polarizer plate 103 as well as the liquid crystal cell 108 and thus to prevent deterioration of the liquid crystal and the polarizer plate due to the heat.
Further, a dust-proofing filter 401 for preventing intrusion of the dust is disposed in front of the cooling fan 400 of the liquid crystal projector as shown in FIG. 1, so as to prevent deterioration of quality of the projected image due to the dust.
However, it has not been possible to completely eliminate the dust that has entered the liquid crystal projector during the production thereof and the dust that unavoidably enters when the lamp as the illumination means is exchanged. Accordingly, the dust that has entered the liquid crystal projector flies up due to the air pressure of the cooling fan or to the vibration or drop of the projector during transportation, and adheres to the surface of the liquid crystal cell. When the dust adheres to the liquid crystal cell, even if the dust is small, it is magnified by the optical system as the projection lens and deteriorates the quality of the projected image.
Once adhered to the surface of the liquid crystal cell, the dust cannot be removed easily because the the liquid crystal cell is sealed in the casing of the liquid crystal projector, and the liquid crystal projector cannot be disassembled without using a high level of skill and the jigs used by the manufacturer of the liquid crystal projector. Further, the liquid crystal cell may be damaged when removing the dust, and this damage results in a drop in the quality of the projected image.
Accordingly, no means has been available for effectively preventing adhesion of the dust in the liquid crystal projector to the surface of the liquid crystal cell, or a method of easily removing the dust when the dust adheres to the surface of the liquid crystal cell, and users must ask the manufacturer to disassemble the liquid crystal display unit. Thus, not only the cost is high, but use of the liquid crystal projector is curtailed.
On the other hand, the liquid crystal cell, too, has a problem in that its temperature characteristics change when the temperature rises and a drop in contrast occurs. This problem must be solved together with the problems described above.